At Ballybunion Cashen, many of the green sites are around 3,000 sq feet... When you add in wind, firm conditions and surrounding contour, it doesn't take much to work out that this is the main reason the course is maligned by some...
Obviously "small" depends on all the variables of the site but fairly flattish greens in softer conditions with sympathetic surrounding contour, I'd go with 3,000 sq feet as still too small... Firmer conditions and undulating surfaces I'd say 4,500 sq feet is pushing it....